[cauterized]

Yes, you should get someone to do a code review. Ideally, you should make this part of the ongoing process so that design and architecture problems can be caught early, while its easier to unravel them.

For the code review, plan to hire someone mid-level to senior - with at least 5 years iOS development experience and several apps shipped to the App Store. (And check those app reviews - don't hire someone whose app crashes a lot!) That will mean a higher hourly rate, but they'll know best practices better. You can hire them on a per-hour or a per-project basis. The number of hours will depend on the size of the codebase.

Code is harder to read and understand than it is to write. It takes far longer to read a chunk of code than it does to read a similar number of characters of non-fiction prose. For a small codebase, the reviewer should be able to finish in 1-2 weeks. For a sprawling one, it could take a couple months. Get an estimate from anyone you consider hiring.

You've already gotten decent advice on NDAs. My experience is that they're not really enforceable in many common outsourcing destinations for both legal and cultural reasons. And that in the US and other "western" cultures you don't really need them if you're dealing with someone operating in good faith. But in either case they can't hurt as a CYA move.

Good luck!